Telephony



Sept 13 1938 F; DE FREMERY 2,29,990

TELEPHONY Filed Sept. 19, 1936v Patented Sept. 13, 1938 UNITED STATES PATENT OFFICE TELEPHONY Application September 19, 1936, Serial No. 101,669 In Germany September 19, 1935 3 Claims.

My invention relates to telephony and more particularly to duplex communication systems using a forked circuit arrangement.

Forked circuit arrangements are commonly used in duplex communication systems for connecting a. subscriber line to a transmitting channel and a receiving channel using a double line, such as the four-wire arrangement used in line telephony or in conference telephony. However, these channels may'v also be constituted partly by wireless transmitting installations and my inVen tion includes such installations.

-To overcome back-coupling such forked arrangements are provided with so-called line balances which should match as closely as possible the apparent resistanceof the subscriber line; the degree of freedom from back-coupling depending upon the quality of these line balances. As in most cases such line balances do not fully meet the requirements, it has been suggested to increase the degree of freedom from back-coupling by adding back-coupling blocking means which, in duplex systems, serve to block one transmitting direction when speech current flows in the other transmitting direction.

The object of my invention is to overcome the above diiiiculties and to provide a forked circuit system having back-coupling blocking means, which systemis completely free from back-coupling and does not employ line balances.

In accordance with my invention, I provide the system with two additional impedances which form with the input impedance of the transmitting channel and with the output impedance of the receiving channel, a bridge arrangement havingv as a diagonal the subscriber line in series with another impedance. With this arrangement, the alternatingcurrents coming from the subscriber line produce equal and opposite voltages across the additional impedances, whereas the alternating currents coming from the receiving channel produce rectied'voltages across both of the additional impedances; the A. C. voltages across the impedance connection in series withV the subscriber line and those produced across the two additional impedances controlling the back-coupling barrier in opposite directions.

In order that my invention may be clearly understood and readily carried into effect, I s hall describe the same more fully with reference to the accompanying drawing in which:

Figure 1 is a schematic Wiring diagram of a.

conference telephone systemembodying my invention, and 7 Figure 2 is a schematic wiring diagram of the bridge arrangement used in Fig. 1.

Referring to Fig. l, the reference number I designates a conference room in which are 1ocated a microphone 2 and a loudspeaker 3 connected through a forked arrangement to the subscriber lines 8. and 9. For this purpose the loudspeaker 3 is connected through conductors 6 6, to an amplier e and` hence through conductor and an impedance I2 to the subscriber line B, and through conductor`26 and an impedance II to point 24 connected 'to thesubscriber line 9. Similarly, microphone 2 is connected through conductors 'I- to an amplifier 5, and hence through a conductor 2l and the impedance I2 to the' subscriber line 8, and through conduc tor 28 and an impedance III to point 24. As the constructionA of such amplifiers is well known, detailed discussion of the same is believed unnece'ssary,

The system also includes a back-coupling sup-` pressor comprising a grid-controlled amplifying tube I3, and a relay I4 'whose actuating coil is connected in the anode circuit of tube I3 with one terminal connected to the plate of the tube and its other terminal connected through a battery 3l tothe cathode. The relay I4 has a normally-open contact 20 adapted to shunt the lines 5 a-nd a normally-closed contact 2l' adapted toshunt the lines 1.

Connected in series across the grid and cathode of tube I3 are two resistances I8 and I9 shunted by condensers 22 and 23 respectively. The resistance I8 is connected through a rectifier I6 to the secondary winding of a transformer 29 having a primary winding connected across the impedance I2. Similarly, resistance I9 is connected through a rectifier I'I to the secondary winding of a transformer 3Q Whose primary winding has its ends connected across impedances III and I I and its centertap connected to point 24.

As indicated in Fig. 2, ZS representing the inputl impedance of the transmitting circuit and Ze representing the output impedance of the receiving circuit, formV together with impedances I0 and II, a bridge arrangement having a diagonal comprising the series connection of the subscriber lines B and .9 and the impedance I2. The impedances I0 and I I are given such Values that speech currentscoming from the subscriber lines 8, 9, will produce equal and opposite A. C. voltages in the impedances III and II, whereby no A. C. Voltage will be applied to rectifier I'I and hence will cause no voltage drop across resistance I9. However, when the subscriber is talking at the end of line 8, an A. C. voltage is produced across impedance I2 and applied to rectifier I6 to thereby produce a D. C. voltage drop across resist-Y ance I8.

This Voltage drop acrossv resistance I8 negatively biases the grid of tube I3 to such an extent that the anode current is reduced to thereby release relay I4. With relay I4 released contact 2l shunts the lines 1 of the receiving circuit and contact 20 opens to remove the shunt from lines 6 of the transmitting circuit. The speech currents coming from the subscriber lines are then ampliiied by amplifier 4 and actuate the speaker 3.

If the microphone 2 is used, the speech oscillations thereof are amplified by amplifier 5, reach the forked arrangement and produce an A. C. voltage across impedances IU, II and I2. appears from Fig. 2, the A. C. voltages across impedances I0 and II do not neutralize each other,

but produce an A. C. voltage which is rectied by rectifier Il to produce a D. C. voltage drop across resistance I9 to thereby bias the grid of tube I3 more positively. The A. C. voltage produced across impedances I2 by these speech oscillations is rectied by rectifier I6 and produces across resistance I8 a D. C.'voltage drop Which negatively biases the grid of tube I3. By giving the impedances I0, II and I2, the resistances I8 and I9, and the transformers 29 and 30, suitable electrical values, it can be insured that the voltage drop across resistance I9 is equal to or greater than the voltage drop across resistance I8 whereby the bias of the grid of tube I3 does not vary or become more positive. Under these conditions the relay I4 remains closed and contact 2D shunts the lines 6 of the transmitting channel whereas contact 2IY is open to remove the shunt from lines 1.

From the above it appears that, when speaking on the subscribers lines, the lines 6 of the transmitting channelare not shuntedand the lines 'I are shunted to block the receiving channel, and that, when using the microphone 2, the lines 6 of the transmitting channel are shunted and the lines 'I of the receiving channel are open. In either case, any undesirable back-coupling is completely avoided.

While I have described my invention in conv nection with specific examples and applications,

I do not Wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:-

1. A duplex communication system comprising a subscriber line, a receiving channel and a transmitting channel connected to said subscriber line in a fork arrangement without the use of a line balance, a back-coupling suppressor for short-circuiting either the receiving or the transmitting channel, and means for controlling said back-coupling suppressor comprising an auxiliary impedance in series with the subscriber line and serving to produce a controlling voltage, a supplementary impedance in said receiving channel, a second supplementary impedance in said transmittingv channel.' said supplementary impedances being connected in series and serving to produce a controlling voltage, the alternating currents coming from the subscriber line producing equal and opposite voltages in said supplementary impedances, said supplementary impedances forming with the input impedance of the transmitting channel and the output impedance of the receiving channel a bridge arrangement having a diagonal comprising the subscriber line in series with said auxiliary impedance, means for rectifying the voltage produced across the auxiliary impedance to produce a rectied voltage whichV controlls the suppressor in one direction, and means for rectifying the Asv voltage produced across the series-connection of the supplementary impedances to produce a rectified voltage which controls the suppressor in the opposite direction.

2,. A duplex communication system comprising a subscriber line, a receiving channel and a transmitting channel connected to said subscriber line in a fork arrangement without the use of a line balance, a back-coupling suppressor for short-circuiting either said receiving or transmitting channel, and means for controlling said back-coupling suppressor comprising an auxiliary impedance in series with the subscriber line and serving to produce a controlling voltage, a supplementary impedance in said receiving channel, and a second supplementary impedance in said transmitting channel, said supplementary impedances being connected in series and serving to produce a controlling voltage, the alternating currents coming from the subscriber line producing equal and opposite voltages in said supplementary impedances, and the alternating current coming from the receiving channel producing additive voltages in said supplementary impedances, said supplementary impedances forming with the input impedance of the transmitting channel and the output impedance of the receiving channel a bridge arrangement having a diagonal comprising the subscriber line in series with said auxiliary impedance, means for rectifying the voltage across said auxiliary impedance to produce a rectied voltage Which controls the suppressor in one direction, and means for rectifying the voltage across the series connection of the supplementary impedances to produce a rectified voltage which controls the suppressor in the opposite direction.

3. A duplex communication system for conference telephonycomprising a receiving channel, a subscriber line, a transmitting channel, said receiving and transmitting channels being connected to said subscriber line in a fork arrangement Without the use of a line balance, a back-coupling suppressor for short-circuiting either said receiving or transmitting channel comprising a grid-control tube, and a relay having an actuating coil connected in the plate circuit of said tube, and contacts in one position shunting said transmitting channel and in the other position shunting said receiving channel, and means for biasing the grid of said tube comprising an auxiliary impedance in said subscriber line, a supplementary impedance in said transmitting channel, a second supplementary impedance in said receiving channel, said supplementary impedances forming with the input impedance of the transmitting channel and with the output impedance of the receiving channel a, bridge arrangement having a diagonal comprising the subscriber line in series with the auxiliary impedance, two resistances connected in series in the grid circuit of said tube, a rectifying circuit connecting one of said resistances across said auxiliary impedance, and a second rectifying circuit connecting the second resistance across the series connection of said supplementary impedances and including a transformer having a primary winding provided with a mid-tap connected between said supplementary lmpedances.

- FRANK DE FREMERY. 

